What is the best way to to add a new number w/ 2 extensions to current account?

Did you mean to share 1 phone number with 2 users (Ext 1 and Ext 2)?  If yes, I don't think it is possible except using the main company number and the extension number e.g. 1234567890*101, 1234567890*102

But if you want to add a direct number to each extension, just log in for your production account: https://service.ringcentral.com. For the sandbox account https://service.devtest.ringcentral.com. Then select a user (if you don't have a user other than 101, click the "+ Add User" button) and select the "Phones & Numbers" tab then "Add Direct Number". then follow the instruction to assign the number to the extension.

Let me know if this is not what you really want.